The present invention relates to a process for polymerizing ethylene.
It is known to polymerize ethylene by means of metallocene catalysts. Such processes result in the manufacture of polyethylenes having a low bulk density (BD).
Moreover, the use of antistatic agents in industrial polymerization processes is well known. These antistatic agents reduce electrical charges and thus prevent the formation of agglomerates and of deposits on the walls of the polymerization reactors. Patent applications WO 99/61486 and WO 96/11960 disclose processes for polymerizing ethylene using a supported metallocene, an aluminoxane, a trialkylaluminum and a nonionic antistatic agent chosen from diethoxylated tertiary alkylamines, which do not cause coating. Patent application EP 0 803 514 discloses a process for (co)polymerizing propylene using a supported metallocene catalyst, an aluminoxane, a trialkylaluminum and an ionic antistatic agent, which does not cause coating nor the formation of agglomerates.
A process has now been discovered for polymerizing ethylene which makes it possible to obtain polyethylenes of high bulk density with a high catalytic activity and without the walls of the reactor being fouled.
For this purpose, the present invention relates to a process for manufacturing ethylene homopolymers or ethylene copolymers comprising at least 90 mol % of units derived from ethylene, in which process ethylene, and optionally the other monomers, are brought into contact, under polymerizing conditions, with a catalytic system comprising:
(a) a catalytic solid comprising a metallocene of a transition metal of Groups 4 to 6 of the Periodic Table, which contains at least one cyclopentadiene ligand, possibly substituted, deposited on a support;
(b) at least one organoaluminum compound chosen from compounds satisfying the general formula (1)
AlTx(Yxe2x80x2)yXxe2x80x2zxe2x80x83xe2x80x83(1)
in which:
T is a hydrocarbon group containing from 1 to 30 carbon atoms,
Yxe2x80x2 is a group chosen from xe2x80x94ORxe2x80x2, xe2x80x94SRxe2x80x2 and NRxe2x80x2Rxe2x80x3, where Rxe2x80x2 and Rxe2x80x3 represent, independently, a hydrocarbon group containing from 1 to 30 carbon atoms,
Xxe2x80x2 is a halogen atom,
x is a number satisfying the condition
0 less than xxe2x89xa63,
y is a number satisfying the condition 0xe2x89xa6y less than 3,
z is a number satisfying the conditions 0xe2x89xa6z less than 3 and x+y+z=3; and
(c) at least one ionic antistatic agent.
According to the present invention, the expression xe2x80x9cprocess for polymerizing ethylenexe2x80x9d is understood to mean a process for manufacturing ethylene homopolymers and ethylene copolymers comprising at least 90 mol % of units derived from ethylene. The preferred copolymers are those of ethylene with another alpha-olefin comprising from 3 to 8 carbon atoms. Particularly preferred are ethylene/1-butene and/or ethylene/1-hexene copolymers.
The metallocene used in the process according to the present invention is usually chosen from compounds satisfying the formula
Qa(C5H5-d-bR1b)(C5H5-d-cR2c)MeXYxe2x80x83xe2x80x83(2)
in which:
Q represents a divalent linking group between the two cyclopentadiene ligands (C5H5-d-bR1b) and (C5H5-d-cR2c);
a equals 0 or 1;
b, c and d are integers satisfying the conditions 0xe2x89xa6bxe2x89xa65, 0xe2x89xa6cxe2x89xa65 and 0xe2x89xa6dxe2x89xa65 when a equals 0, and 0xe2x89xa6bxe2x89xa64, 0xe2x89xa6cxe2x89xa64 and 0xe2x89xa6dxe2x89xa64 when a equals 1;
R1 and R2 each represent hydrocarbon groups containing from 1 to 20 carbon atoms and able to be linked to the cyclopentadiene ring in the form of a monovalent group or able to be connected to each other so as to form a ring adjacent to the cyclopentadiene ring, halogen atoms, alkoxy groups having from 1 to 12 carbon atoms, silicon-containing hydrocarbon groups of formula xe2x80x94Si(R4)(R5)(R6), phosphorus-containing hydrocarbon groups of formula xe2x80x94P(R4)(R5), nitrogen-containing hydrocarbon groups of formula xe2x80x94N(R4)(R5) or boron-containing hydrocarbon groups of formula xe2x80x94B(R4)(R5) in which R4, R5 and R6 represent hydrocarbon groups containing from 1 to 24 carbon atoms, as long as when b, c or d equals 2 or more and/or a plurality of groups R1 or R2 exist, the latter may be identical or different;
Me represents a transition metal of Groups 4 to 6 of the Periodic Table; and
X and Y, which are identical or different, each represent a hydrogen atom, a halogen atom, a hydrocarbon group, an alkoxy group, an amino group, a phosphorus-containing hydrocarbon group or a silicon-containing hydrocarbon group having from 1 to 20 carbon atoms.
The preferred transition metal compounds of formula (2) are generally such that:
Q represents an alkylene group containing 1 or 2 carbon atoms, possibly substituted with alkyl or aryl groups containing from 1 to 10 carbon atoms, or a dialkylgermanium or dialkylsilicon group containing from 1 to 6 carbon atoms;
a equals 0 or 1;
b, c and d are integers satisfying the conditions 0xe2x89xa6bxe2x89xa65, 0xe2x89xa6cxe2x89xa65 and 0xe2x89xa6dxe2x89xa65 when a equals 0, and 0xe2x89xa6bxe2x89xa64, 0xe2x89xa6cxe2x89xa64 and 0xe2x89xa6dxe2x89xa64 when a equals 1;
R1 and R2 represent alkyl, alkenyl, aryl, alkylaryl, alkenylaryl or arylalkyl groups containing from 1 to 20 carbon atoms, it being possible for several groups R1 and/or several groups R2 to be linked to each other so as to form a ring containing from 4 to 8 carbon atoms;
Me is zirconium, hafnium or titanium; and
X and Y represent halogen atoms or hydrocarbon groups chosen from alkyls, aryls and alkenyls containing from 1 to 10 carbon atoms.
Particularly preferred are metallocenes of formula (2) in which Q is a linking group chosen from dimethylsilyl and diphenylsilyl, ethylene and methylenes and ethylenes substituted with alkyl or aryl groups containing from 1 to 8 carbon atoms. Particularly suitable compounds of formula (2) are compounds in which the ligands (C5H5-d-bR1b) and (C5H5-d-cR2c) are chosen from cyclopentadienyls, indenyls and fluorenyls, these possibly being substituted. The catalytic solid (a) usually also includes an activator. The activator is generally chosen from aluminoxanes and ionizing agents.
The term xe2x80x9caluminoxanesxe2x80x9d is understood to mean compounds satisfying the formula R7xe2x80x94(AlR7xe2x80x94O)mxe2x80x94AlR72 and the cyclic compounds satisfying the formula (xe2x80x94AlR7xe2x80x94Oxe2x80x94)m+2 in which m is a number from 1 to 40 and R7 is an alkyl or aryl group containing from 1 to 12 carbon atoms. The preferred compounds are chosen from methyaluminoxanes, ethylaluminoxanes, isobutylaluminoxanes and mixtures thereof, and more particularly those in which m is a number from 2 to 20. Most particularly preferred is methylaluminoxane (MAO) in which m is a number from 10 to 18.
The term xe2x80x9cionizing agentsxe2x80x9d is understood to mean compounds comprising a first part which has the properties of a Lewis acid and is capable of ionizing the metallocene and a second part which is inert with respect to the ionized metallocene and is capable of stabilizing it. As examples of such compounds, mention may be made of triphenylcarbenium tetrakis(pentafluorophenyl)borate, N,N-dimethylanilium tetrakis(pentafluorophenyl)borate, tri(n-butyl)ammonium tetrakis(pentafluorophenyl)borate, tri(pentafluorophenyl)boron, triphenylboron, trimethylboron, tri(trimethylsilyl)boron and organoboroxines.
The amount of activator in the catalytic solid depends on the type of activator used. When the activator is an aluminoxane, the amount of aluminoxane is usually such that the atomic ratio of aluminum coming from the aluminoxane to the transition metal coming from the metallocene is from 2 to 5000. Preferably, this ratio is at least 5, more particularly at least 10. Good results are obtained when this ratio is at least 20. Usually the aluminoxane is employed in amounts such that the aluminum/transition metal atomic ratio is at most 2000, more particular at most 1500. Atomic ratios of aluminum coming from the aluminoxane of [sic] the transition metal of at most 1000 are most particularly preferred. Ratios of at most 300 give good results. When the activator is an ionizing agent, the amount of ionizing agent is usually such that the molar ratio of the ionizing agent to the metallocene is from 0.05 to 50. Preferably, this ratio is at least 0.1 and more particularly at most 20.
The catalytic solid (a) contains a support. The support may be any known organic or inorganic support allowing the metallocene and possibly the activator to be supported. As nonlimiting examples of inorganic supports, mention may be made of talc or inorganic oxides-such as silicas, aluminas, titanium, zirconium or magnesium oxides, or mixtures thereof. Such supports have been disclosed in patent application EP 0 206 794 for example. The organic supports are usually chosen from among porous polymeric supports, and more particularly from among polyolefin supports such as those disclosed in patent application EP 1 038 883. Inorganic supports are preferred within the context of the present invention. Silica is particularly preferred.
The catalytic solid (a) used in the process according to the invention may be obtained by various methods. In general, support particles are brought into contact with a solution containing the activator in order to obtain a suspension which is then evaporated. The metallocene may be introduced into the suspension described above. It may also have been incorporated into the support before it is brought into contact with the activator. Finally, it may be brought into contact with the support particles containing the activator.
The catalytic solid (a) employed in the process according to the present invention generally contains from 0.001 to 5 g of metallocene per gram of support. Preferably, the concentration of metallocene is at least 0.005 g and more particularly at least 0.01 g per gram of support. Amounts of metallocene less than or equal to 3 and preferably less than or equal to 1 g per gram of support give good results.
According to a variant of the process according to the invention, a catalytic solid (a) is used that has been subjected to a preliminary polymerization during which it is brought into contact with an alpha-olefin, under polymerizing conditions, so as to form from 0.01 to 50 g of polyolefin per g of catalytic solid. The alpha-olefin used during the preliminary polymerization step is advantageously chosen from among alpha-olefins containing from 2 to 4 carbon atoms.
The catalytic system also includes at least one organoaluminum compound (b) satisfying the general formula (1). The organoaluminum compound is preferably chosen from among trialkylaluminums of formula AlT3, and more particularly from among those in which each T represents, independently, an alkyl group containing from 1 to 20 carbon atoms. Particularly preferred is a trialkylaluminum in which T is an alkyl group containing from 1 to 6 carbon atoms, such as trimethylaluminum (TMA), triethylaluminum and triisobutylaluminum (TIBAL).
The amount of organoaluminum compound (b) employed in the process according to the invention is in general such that the atomic ratio of the aluminum coming from the organoaluminum compound (b) to the transition metal coming from the metallocene is from 10 to 50 000. Preferably, this ratio is at least 20, more particularly at least 30. Goods results are obtained when this ratio is at least 40. Usually the organoaluminum compound (b) is employed in amounts such that the aluminum coming from the organoaluminum compound/transition metal coming from the metallocene atomic ratio is at most 20 000 and more particularly at most 17 000. Ratios of at most 15 000 give good results.
The catalytic system used in the process according to the invention also includes at least one ionic antistatic agent (c). Within the context of the present invention, the ionic antistatic agents are generally chosen from among those containing a long hydrophobic chain. Preferably, ionic antistatic agents comprising at least one hydrocarbon group containing from 6 to 35 carbon atoms is used, this group being possibly substituted.
According to a first variant of the process according to the invention, the antistatic agent is chosen from among cationic antistatic agents and more particularly from among quaternary ammonium salts represented by the general formula A1A2A3A4NX1 in which A1, A2, A3 and A4 represent, independently, a hydrocarbon group containing from 1 to 35 carbon atoms and at least one of A1, A2, A3 and A4 is a hydrocarbon group containing from 6 to 35 carbon atoms, and X1 is a halogen atom. Quaternary alkylammonium salts containing at least one alkyl group containing from 6 to 35 carbon atoms are preferred. Quaternary alkylammonium salts containing at least one alkyl group containing from 6 to 35 carbon atoms derived from a fatty acid give good results. As a nonlimiting example of a quaternary ammonium salt, mention may be made of dicocoalkyldimethylammonium chloride. The product commercially available under the name CHEMAX(copyright) X-997 is particularly preferred.
According to a second variant of the process, the antistatic agent is chosen from among anionic antistatic agents and more particularly from among sulfonic acids comprising at least one hydrocarbon group containing from 6 to 35 carbon atoms, this group possibly being substituted. Sulfonic acids comprising a hydrocarbon group, preferably an aryl group, containing from 6 to 18 carbon atoms and substituted with at least one alkyl group containing from 6 to 16 carbon atoms give good results. As a nonlimiting example of a sulfonic acid, mention may be made of dinonylnaphthalenesulfonic acid. The product sold by the company Octel under the name STADIS(copyright) 450 is particularly preferred.
The amount of antistatic agent employed in the process according to the invention is in general such that the molar ratio of the antistatic agent (c) to the organoaluminum compound (b) is less than 0.5. Preferably, the molar ratio of the antistatic agent to the organoaluminum compound is less than 0.2. Molar ratios of less than 0.1 are particularly preferred. The amount of antistatic agent is such that the molar ratio of the antistatic agent (c) to the organoaluminum compound (b) is in general at least 0.001. Preferably, this molar ratio is at least 0.002, more particularly at least 0.003.
In the process according to the invention, it is advantageous to prepare a premixture comprising at least the organoaluminum compound (b) and the antistatic agent (c), before the catalytic solid (a) is added thereto.
The polymerization process according to the invention may be carried out continuously or batchwise, by whatever known process. The polymerization process is preferably carried out in suspension in a hydrocarbon diluent. The hydrocarbon diluent is generally chosen from among aliphatic hydrocarbons containing from 3 to 10 carbon atoms. Preferably, the diluent is chosen from among propane, isobutane, hexane or mixtures thereof.
The temperature at which the polymerization process according to the invention is carried out is generally from xe2x88x9220xc2x0 C. to +150xc2x0 C., usually from 20 to 130xc2x0 C. The polymerization temperature is preferably at least 60xc2x0 C. Preferably, it does not exceed 115xc2x0 C.
The total pressure at which the process according to the invention is carried out is in general chosen to be between atmospheric pressure and 100xc3x97105 Pa, more particularly between 10xc3x97105 and 55xc3x97105 Pa.
The molecular mass of the polymers manufactured according to the process of the invention may be controlled by addition of one or more agents for controlling the molecular mass of polyolefins, such as more particularly hydrogen.
In a variant of the process according to the invention, the process comprises a first polymerization step, separate from the preliminary polymerization step (described above in relation to the catalytic solid) and called prepolymerization step, during which from 1 to 1000 g of polymer per g of catalytic solid are formed. The amount of prepolymer formed in this prepolymerization step is advantageously at least 3 g per g of catalytic solid. Good results are obtained when the amount of prepolymer is at most 700 g per g of catalytic solid. In general, the prepolymerization step is carried out at a temperature from 0 to 60xc2x0 C.
The process according to the invention makes it possible to obtain catalytic activities considerably higher than in the process with no ionic antistatic agent, without the walls of the reactor being fouled, while at the same time giving polyethylenes having a higher bulk density (BD). Obtaining polymers having high BDs has the advantage of increasing the production capability of polymerization plants and of increasing storage and transport capabilities.
The following examples serve to illustrate the invention. The methods for measuring the parameters mentioned in the examples and the units expressing these parameters will be explained below.
The catalytic activity is characterized by the amount of polyethylene formed during polymerization trials and is expressed in kg of polyethylene per mmol of transition metal coming from the metallocene employed, per hour of polymerization and per 105 Pa. In examples 10 to 13R, the catalytic activity is assessed indirectly from the determination by gas chromatography of the residual ethylene in the gas leaving the reactor.
The BD of the polyethylene obtained is expressed in kg/m3. The BD of the polyethylene is measured under free flow using the following operating method: the polyethylene coming from the polymerization process is poured into a cylindrical container of 50 cm3 capacity, taking care not to compact it, from a hopper whose lower edge is placed 20 mm above the upper edge of the container. The container filled with the powder is then weighed, the tare is deducted from the recorded weight and the result obtained, expressed in kg, is multiplied by 20 000 so as to express the BD in kg/m3.
In examples 1 to 9R, the concentration of antistatic agent is expressed in ppm with respect to isobutane.